Polyperfluorochloroethylene and lead dioxide



Uni e-tam? Patent 2,874,144 POLYPERFLUOROCHLOROETHYLENE AND,

LEAD DIOXIDE ,Hanford', Short HillsiN. J., assignor, by mesne 40mins. cl. 260- 4515) This' invention relates to the treatment of perfluorochloroolefin polymers. In one aspect, the invention'to the stabilization of polymers of trifluorochloroethylene, In another aspect, the invention relates to a method to]; improving" heat aging characteristics of thermoplastic polymers of trifluorochloroethylenel Polymers of trifluo'r ochloroethylene are prepared by the polymerization of the rnoriomer triflu'orochloroethylene. The preparation. of monomeric trifiuorochloroethyleriei may be accomplished by various methods. One method comprises dephl oi'inating trifluorotrichlo roe'thane, commercially available as Freon 113, under suitable conditioiis of dechldrinatioiiin the presence of a suitablesolvent, such as methyl alcohol, with a de halogehatihg agent, such as zincdust,1t'o produce-an effluent comprising the mo omer,- unreac'ted trifluorotriclilor'oetharie ahd solveht, I Another method comprises dechlorinating trifluorotrichloroethane by. passing said material over a eat'alyst selectedfrom the group 1B and group VIII metals, and combinations thereof, in the presence of hydrogen, at a temperature above about 200 C The elfiuent obtained from either of the above outlined processes is then passed to a suitable fractional distillationsystem in which substantially pure trifluorochloroethylene is recovered. It usually preferred to conduct distillations in the presence'of a terpene compound at a temperature above about 25 C. since this treatment'removes impurities which are detrimental to the production ofhigh molecular weight polymers. The terpene treatment step may be conducted with the first distillation step or at any convenient time prior to the introduction of the monomer into the polymerizationzone.

'The monomer, obtained as described above, is pol'y merized under suitable polymerization conditions with or Without the presence of a suitable catalyst or; promoter. Such conditions may comprise the use of a suitable cata-, lyst, such as bis-trichloroacet'yl peroxide, dissolved in a suitable solvent such a's"trichlorofiuorom'ethane at a temperature between about '20 C. and about 50 C; An

additional polymerizatioh process employing substantially the sarne conditions as described above maybe used. In this process a dilfererit type of polymerization reactor is employed and the'polymer is formed as a suspension in the monomer and is recovered therefrom by evaporation. s

The polymer produced by either of the above described processes is characterized by wholly different chemical and physical characteristics than otherwise similarbut non-fiuorinated polymeric materials. The chemical resistance of polymers of trifluorochloroethylene is such, that they will withstand exposure to a wide variety of oxidizing, reducing and solvent-type materials, such as fuming nitric acid, concentrated sulfuric acid, hydrazine, hydrogen peroxide, acetone, aniline, etc. Phys ically, the, thermo plastic polymers possess high heat stability, tensile strength, etc. In addition to the above, the thermoplastic polymers are readily molded into varions useful articles, such asvalv es, gaslrets; etc. using? standard equipment but with modified operating condi-- tions.

7 However, when polymers of trifluorochloroethylene;

aremaintained at elevated temperatures for prolonged. periods of time, degradation occurs. This degradation is, inreality, a thermal cracking of the polymer chain with a consequent reduction in molecular weight of the polymet and is evidenced, in the case of thermoplastic polymers, by a reductionin the ZST of the polymer. Uri-f fortunately,mo1ding temperatures so closely approach the degradationfltemperature that under ordinary conditions it is extremelydifiicult to mold the polymer with-'- out some degradation, however slight.

It is ail object of this, invention to provide a process for" improving the thermal stability of perfluorochloroolefin, polymers. h H p It is another object of thisinvention to pro vide a means forstabilizing polymers of trifluorochloroethylene.

It is one of the more particular objects of this invention to improve the thermalstability of polymers of trifluoro'chloroethylene. v v It is another of the particular aspects of this invention to provide a" means for molding polymers of trifluorochloroethylene at temperatures approaching decomposition temperature. e 3 Various other objects and advantages of the present iii-' vention will become apparent to those skilled in the art on reading the accompanying description and disclosure.

Generally, the above objects are accomplished'by admixing with, and intimately dispersingwithin the polymer a quantity of lead dioxide which functionsas a stabilizing agent andiniproves the'thermal stability o f the perfiuorochlorool'e'fin polymers andparticularly the polymers of trifiuorochloroethylene, p

Stabilization of the polymer is effected by admixing with,'and intimately dispersirig within the polymer be tween about 0.05ai1'cl about 20 parts of lead dioxide er parts of polymer by weight, and preferably between about 0.1 and about 10 parts of lead dioxide per lQQ parts of the polymer. The exact concentration of the stabilizing agentlemployed, will be determined by the use for which the polymer is intended. Thus, When the polymer is to be used as a'molding powder, itis preferred to use between about'0.1 andabout 3 parts of the stabiliz ing agent per 100 parts of po'lymer, whereas when used in the form of a dispersion; for example, in xylene, diisobutyl' ketone', aniyl' acetate, water, acetone, etcl, it is preferred to use between about 3 and about 7 parts 0 lead'dioxide per 100 parts'of polymer.

The polymers which" are particularly suited to the process of this invention are the high molecular weight,; normally solid thermoplastic polymers. Since the de' termination of molecular weight of the perfluorochloro olefin polymers, such as polymers of trifluorochloroethylene, is tedious and expensive, it has become an ac cepted practice to express the molecularweig'ht of the polymer in termsof its ZST value which is dependent, onthe molecular weight; Thus, homopolymers of trifluorochloroethylene which are particularly suited. to the process of this invention, have a ZST value of above about This invention also contemplates the treatment of co polymers of trifluorochloroethylene' containing below about 70 mol percent of fluorine containing olefins, such ethylene. be treated, may contain up to about 25jpercent of a fluorinated plasticizer. The fluorinatedi plasticizers are the lower members of the saturated perfluor ochlorocar bon series, for example, polyu'ifluorochloroethylene in the oil to wax range. The addition of plasticizer to the polymer in some instances, advantageously modifies the characteristics of the polymer, for example, by increasing its pliability, etc.

Admixing of the stabilizing agent with the polymer may be effected in a 'variety of ways although usually the standard mechanical techniques are preferred. The method to be employed in effecting the admixture is based on the particle size of the polymer and the stabilizing agent. Thus, if both the polymer and stabilizing agent are in finely-divided form, conventional tumbling type mixers, such as a barrel mixer, conical mixer and mushroom mixer may be employed. If the polymer and stabilizing agent are not already finely-divided, then they should be ground in order to secure thorough admixture. Although the grinding of the stabilizing agent and of the polymer may be effected separately, it is preferred that they be ground together since the presence of the stabilizing agent will also retard thermal degradation which is apt to result during the grinding operation. Grinding of the po lymer and stabilizing agent may also be effected in conventional equipment such as ball and pebble mills, colloid mills and hammer mills. A particularly suitable piece of grinding equipment is the'Mikropulvarizer, a hammer type mill which functions by forcing the material being ground through an appropriate sized screen by means of a hammer action.

For purposes of this invention, the polymer and stabilizing agent are considered as being finely-divided when about 100 percent passes through a #3 /z (0.223) sieve. Preferably, at least 95 percent passes through a #7 (0.11) sieve, and still more preferably at least 95 percent passes through a #12 (0.066") sieve. Admixing is more quickly effected and degradation of the polymer during the grinding operation prevented, when the stabilizing agent is in a more finely-divided form initially. Preferably, 100 percent of the stabilizing agent should pass through a #270 (0.0021") sieve, and still more preferably at least 98 percent through a #325 (0.0017") sieve. The sieve sizes given above are in the U. S.

standard series, the number in parentheses is the sieve opening in inches.

In a preferred method of operation between about 30 and about 70 percent of finely-divided lead dioxide is dry-blended with the polymer. The dry-blended powder is then put through a Mikropulvarizer fitted with between about a 0.02" and about a 0.2" screen, preferably with a 0.06" screen to insure thorough mixing. The stabilized concentrate is then dry-blended with an additional quantity of unstabilized polymer to reduce the concentration of stabilizing agent to the desired level. This finelydivided stabilized polymer powder, has a low apparent density and is suitable for use in certain types .of molding operation, such as compression molding. In other types of molding operations, such as extrusion and injection molding, a high density molding powder is preferred. The low density molding powder is converted to a high density powder by passing it through an extruder and a multiplicity of rod dies. A rotating chopper in front of the die cuts the extruded rod into granules of a. desired size. To minimize polymer degradation during this operation, the temperature is maintained just high enough to compact the powder into granules without converting them completely into homogeneous pellets.

The low density molding powder may also be prepared for use in a dispersion. Conversion of the low density powder to a dispersion grade powder is efiected by grinding in a suitable mill, such as a ball mill, to reduce the particle size to the extent that about 98 percent will pass through a #270 sieve preferably through a #325 sieve. The higher concentration of stabilizing agent which is used with dispersion grade polymers serves to further retard degradation of the polymer during the prolonged grinding operation and also serves to retard degradation during use since dispersion grade polymer is usually used in the form of thin films which are more subject to high temperature effects. After grinding'to the desired particle size, the stabilized polymer is then admixed with a suitable vehicle. Suitable vehicles are those which are generally known as lacquer thinners such as xylene, acetone, di-isobutyl ketone, etc. A particularly suitable vehicle is a mixture of xylene and diisobutyl ketone. In applications where solvents are objectionable, aqueous-dispersions may 'be used. Usually, between about 10 and about 30 percent of the stabilized polymer is added to the vehicle depending on the method of application of the dispersion. Generally, higher concentrations of polymer are used when a dispersion is to be applied by dip or brush techniques, whereas spray techniques usually require a lower solids concentration.

Molding of the low or high density stabilized powder can be accomplished by using suitable molding equipment at temperatures between about 415 F. and 625 F. and a pressure between about 500 and about 25,000 pounds per square inch. Detailed description of preferred molding processes may be found in U. S. Patents Nos. 2,617,149, 2,617,150, 2,617,151, and 2,617,152 issued November 11, 1952, to Louis C. Rubin. When used in the form of a dispersion, the polymer contained in a suitable vehicle such as a mixture of xylene and diisobutyl ketone is applied by spray, brush or dip techniques, and is air-dried or heat-dried at a temperature up to about 302 F. after which it is fused at a temperature between about 464 F. and about 482 F. for a period of time between about one hour and about 24 hours, preferably about 12 hours to insure the production of a homogeneous fihn of high tensile strength. When the stabilized polymer of this invention is fabricated by any of the above described processes, a homogeneous polymer mass is produced which contains, intimately dispersed within its mass, the added stabilizing agent.

As indicated previously, when perfluorochloroolefin polymers are maintained at elevated temperatures for prolonged periods of time (for example during molding), the polymer chain is cracked, thereby reducing the molecular weight of the polymer. Since determination of molecular weights is a tedious process, an empirical test, which tends to reflect the molecular Weight of the sample under test, is used. In this test, the ZST value of the polymer is determined. Briefly, determination of the ZST value is carried out by molding a test strip 0.062"i0.003 thick, 2" long, and ,1 wide. This strip is then notched at the center. The cross-section of the specimen at the notch 'is 0047:0001" wide by 0.062- -0.003". The upper portion of the strip is held in position with a spring clip while a weighted spring clip totaling 7 grams is attached to the other end of the specimen. The weighted specimen is then maintained at a temperature of about 300 C. in a suitable apparatus. The number of seconds required for the sample to break at the notch, is the ZST value. For evaluating the stabilizer of this invention, two ZST values were determined, namely the standard ZST value and the degradation ZST value. The standard ZST value was determined by taking a sample of approximately 40 grams and molding it into a pellet approximately 2" in diameter and A in thickness, by applying 12,000 p. s. i. pressure without heat. The pellet was then placed between two platens which were set to give a sheet 62 mills in thickness. The platens were then placed in a press, preheated for 3 minutes at about 260 C. Without pressure and then heated at about 260 C. for an additional 3 minutes at 20,000 p. s. i. This pressed sheet was removed, cut into a notched ZST strip, and its ZST value determined as described above. The time in seconds required for this strip to break is the standard ZST value. The degradative ZST value was determined by preparing a pellet in the same manner as described for the standard ZST value determinatiomex cept that the pellet was preheated in a press at 300 C. for 5 minutes after which the specimen was heated at 300 C. under a pressure of about 20,000 p. s. i. for an additional 5 minutes. This second sheet was cut into a notched ZST strip and its ZST value determined as described above. The time in seconds required to break this strip at the notch, represents the degradative ZST value. For a fuller and more complete understanding of the method and apparatus used in determining ZST values, reference may be had to application Serial No. 432,681, now U. S. Patent No. 2,729,967, filed May 27, 1954, by H. S. Kaufman and C. R. Giannotta.

EXAMPLE I In order to illustrate the stabilizing effect of lead dioxide, a number of samples were prepared containing lead dioxide in dififerent concentrations. these samples, the quantity of lead dioxide indicated in the table below was added to an amount of a homopolymer of trifluorochloroethylene sufficient to bring the total sample weight up to 40 grams. The polymer and lead dioxide were then blended in a mortar and pestle for about 5 minutes, after which the blend was molded into standard and degradative ZST strips. The standard and EXAMPLE II This example presents data illustrating the use of sodium chlorate as an additive to polytrifluorochloroethylene. In determining the ZST values reported below, the same procedure was employed as described above using sodium chlorate as the additive. The homopolymer of trifluorochloroethylene was from the same batch as that used in Example I. The quantity of sodium chlorate indicated in the table, was added to an amount of polymer suflicient to bring the total sample weight up to 40 grams. Standard and degradative ZST test strips were then prepared and the ZST values determined as described above. These values are reported in the table below. From these values it will be observed that some stabilizing eifect is obtained. However, each of the samples was badly bubbled and the pressings were nonuniform. Consistent results were not obtained as shown by the runs in which 0.04 and 0.4 gram of sodium chlorate were added. In addition, the degradation products obtained with these additives were corrosive and damaged the-metal platens used in pressing the samples.

In preparing Table 2 ZST values Sodium chlorate, g.

Standard Degradative Various alterations and modifications of the invention and its aspects may become apparent to those skilled in the art without departing from the scope of this invention.

Having thus described my invention, I claim:

1. A novel plastic composition consisting of an admixture of a homopolymer of trifluorochloroethylene and between about 0.05 and about 20 parts of lead dioxide per 100 parts of said homopolymer, said homopolymer being inert to chemical cross-linking in the presence of lead dioxide.

2. A novel plastic composition for use in a dispersion consisting of an admixture of a homopolymer of trifluorochloroethylene and between about 3 and about 7 parts of lead dioxide per 100 parts of said homopolymer, said homopolymer being inert to chemical cross-linking in the presence of lead dioxide.

3. A novel plastic composition consisting of an admixture of a plastic homopolymer of trifluorochloroethylene and between about 0.1 and about 3 parts of lead dioxide per 100 parts of said homopolymer, said homopolymer being inert to chemical cross-linking in the presence of lead dioxide.

4. A novel plastic composition prepared from a molding powder consisting of a homogeneous trifluorochloroethylene homopolymer mass and intimately dispersed therein between about 0.1 and about 3 parts of lead dioxide per parts of said homopolymer, said homopolymer being inert to chemical cross-linking in the presence of lead dioxide.

References Cited in the file of this patent OTHER REFERENCES Zapp et al.: Journal of Polymer Science, vol. 9-10, pages 97-113, August 1952. 

1. A NOVEL PLASTIC COMPOSITION CONSISTING OF AN ADMIXTURE OF A HOMOPOLYMER OF TRIFLUOROCHLOROETHYLENE AND BETWEEN ABOUT 0.05 AND ABOUT 20 PARTS OF LEAD DIOXIDE PER 100 PARTS OF SAID HOMOPOLYMER, SAID HOMOPOLYMER BEING INERT TO CHEMICAL CROSS-LINKAGE IN THE PRESENCE OF LEAD DIOXIDE. 